I have an old dipole, center fed with window line, and I want to connect it but I want to run coax from the transmitter out to the feedline.What is the best way to connect the coax to the window feed line ?Should I use a choke balun 1:1 ?

Be aware that you can end up with a *surprising* amount of loss in that coax.

There was a recent QST article recommending coax between the tuner and the ladderline outside that assumed a 10:1 SWR on the coax, but the true SWR can be much, much higher depending on the antenna and length of ladderline.

The SWR at the end of 50 feet of 450 ohm ladder line feeding an 80m dipole on 10m is nearly 39:1, not 10:1, and although the twinlead only has 0.4dB loss, you pick up nearly 3dB extra loss in just 20 feet of RG-8 due to the 39:1 SWR mismatch. You might not expect to lose half your power in 20 feet of coax, but there's a possibility of that with a pretty common configuration.

I had a random antenna fed with about 20 feet of RG-6 type CATV coax once, and I had so much loss in that line on 17m that when I moved the tuner out to the apartment balcony instead of at the shack end of the coax, I got two or three S-units improvement... I could *easily* hear the difference in my receiver.

So if you really, really have to use coax between the tuner and the open wire line, keep it short, and I don't mean 20 feet, I mean 3 feet ;-) Your mileage may vary, of course. Some bands with some antennas will have negligible loss in a couple dozen feet of decent coax, but I've seen a lot of serious *under*estimations of the mismatched line loss floating around out there too.

I used VK1OD's transmission line calculator to calculate the losses in the coax and ladderline and the impedance transformation in the ladderline :

I used 14 foot of window line direct coupled to the RG8X coax (about 35 feet). Made a 1:1 coax balun at the closest I could get to the feed point on a plastic coffee can, 8-10 turns about 8in dia. Works pretty good on my fan dipole.

I use shielded parallel line (SPL) jumpers: 2 coaxes (any impedance) in parallel, using the centers to carry the balanced line straight to the balun, then to the tuner. Some tuners have a 4:1 balun built in.

There's a 2 foot run of SPL from the end of the ladder line to my entrance panel. At the LL/Coax junction, the coax centers go to each LL wire, and the coax shields are soldered together and floating.

The coax shields get grounded when they pass through the UHF bulkhead connectors in the entrance panel.

On the inside, there's an 8 ft SPL jumper to the balun and tuner. At the balun, the coax shields get connected to the tuner's ground terminal. The total coax length is only 10ft. As 'OX reported, this is where the high SWR losses are.

In general, a 4:1 balun works best when multibanding an 80m doublet. I have both 1:1 and 4:1 in a switchable balun and find that 4:1 is better most of the time.

You may have to add or subract some feedline length (coax or LL) to present your tuner with impedances it can match on all bands, 80-10.

I should have explained why the described shielding configuration does not suppress common mode current.

The configuration is a pair of ladder line conductors connect to the inner conductors of two coaxial cables. The shields of the coaxial cables are tied together and not connected to anything else.

It is popularly held that this arrangement effectively shields or contains common mode currents and that there is no external field.

Lets examine how it works:

There is a circuit node (N1) formed where the shields are tied together, the node joins the inner surface of the outer conductor of one coax, the inner surface of the outer conductor of the other coax, and the outer surfaces of the two coaxs.

Note that for coax at HF where skin effect is fully developed, current flowing on the inner surface of the outer conductor is isolated from current flowing on the outer surface of the outer conductor.

If current I1 flows into one of the inner conductors (on the outer surface), then in TEM mode, there will be a current I1 flowing out of the inner surface of the outer conductor into node N1.

If current I2 flows into the other the inner conductors (on the outer surface), then in TEM mode, there will be a current I2 flowing out of the inner surface of the outer conductor into node N1.

By KCL the current flowing into the third conductor (the outer surface of the outer conductors) connected to node N1 is I1+I2.

I1+I2 is the common mode current that was flowing on the ladder line adjacent to the transition, and it now flows on the outer surface of the outer conductors on the coax side of the transition.

The common mode current is unaltered by the shielding configuration. Common mode current is not eliminated, external fields will still exist if there is common mode current on the feedline.

Of course, the shielded coax pair configuration is usually much more lossy than two wire open line.

Another thing to watch out for is the fact that the SWR at the shack end of the line is always less than the SWR at the antenna end, because line loss "hides" the actual mismatch. The lossier the line, the worse the effect. If the line is lossy enough, you can short or open the antenna end of the line and see no worse than 2 or 3 to 1 SWR at the shack end.

1) Passing the SPL through an entrance panel makes it very easy to short conductors to ground using conventional UHF hardware.

2) The SPL jumper can be routed next to metal and other coaxes without concern for imbalance.

3) Connecting the SPL jumper to your tuner provides a safety ground path for the rig to drain static through the shields back to the entrance panel.

4) It's very easy to install conventional coaxial lightning arrestors (if desired) in the SPL jumper right on the entrance panel.

Multibanding through a section of coax definitely increases losses. That's why the emphasis has been to keep the sections short. I believe 10' max is a reasonable compromise that's worth it to allow the above benefits.

My point was about the treatment of the shields where they were left floating, namely at the junction with the ladder line. Shields treated in that way do not perform the shielding function often thought.

Benefit #2 may apply to some extent to the section of 'SPL' between the point where you ground the shields (the entrance panel) and the junction with the ladder line.

None of the other claimed benefits apply to that section.

Yes, an effective current balun is a good countermeasure to help to reduce the level of common mode current on the antenna feedline.

To the OP's original question, there is significant disadvantage in transitioning from ladder line to coax part way along the feedline from a loss point of view (and that depends on the scenario) and arguably, little or no advantage.

The shielding effect, ie the containment of energy to the inside of the shield with negligible external field, depends on the treatment of the ends of the shield. Note that current may be caused in common mode in two wire open line and coax both, as a result of unbalanced coupling to the load and environment.

Coax doesn't magically eliminate external fields in the shack.

The same is also true of shielded two wire line.

A shield is not a shield just by virtue of its name, just like a balanced line is not balanced just by virtue of its name... both are bad terms, outer conductor and two wire open line respectively are better terms that don't engender mythical properties.

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